linux-kernel - Re: [PATCH linux-next v3 2/4] sched/psi: Avoid update triggers and rtpoll

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Message-ID: <ZScQZLTssSfq19Jm@gmail.com>
Date:   Wed, 11 Oct 2023 23:15:16 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     yang.yang29@....com.cn
Cc:     surenb@...gle.com, peterz@...radead.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        mingo@...hat.com
Subject: Re: [PATCH linux-next v3 2/4] sched/psi: Avoid update triggers and
 rtpoll_total when it is unnecessary


* yang.yang29@....com.cn <yang.yang29@....com.cn> wrote:

> From: Yang Yang <yang.yang29@....com.cn>
> 
> When psimon wakes up and there are no state changes for rtpoll_states,
> it's unnecessary to update triggers and rtpoll_total because the pressures
> being monitored by user had not changed.
> This will help to slightly reduce unnecessary computations of psi.
> 
> Signed-off-by: Yang Yang <yang.yang29@....com.cn>
> Cc: Zhang Yunkai <zhang.yunkai@....com.cn>
> Cc: Ran Xiaokai <ran.xiaokai@....com.cn>
> ---
>  kernel/sched/psi.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index be853f227e40..143f8eb34f9d 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -704,11 +704,12 @@ static void psi_rtpoll_work(struct psi_group *group)
>  	}
> 
>  	if (now >= group->rtpoll_next_update) {
> -		update_triggers(group, now, &update_total, PSI_POLL);
>  		group->rtpoll_next_update = now + group->rtpoll_min_period;
> -		if (update_total)
> +		if (changed_states & group->rtpoll_states) {
> +			update_triggers(group, now, &update_total, PSI_POLL);
>  			memcpy(group->rtpoll_total, group->total[PSI_POLL],
>  				   sizeof(group->rtpoll_total));
> +		}

Yeah, so I believe we may have been talking past each other for past 
versions of this patch: why is this patch modifying the order of the 
modification to group->rtpoll_next_update?

It should do the below sequence, nothing more - see the patch attached 
below. This is basically a combination of patches #2 and #3.

And then the final patch removes the now superfluous 'update_total' 
parameter, which is always true.

Here are the commits I applied to tip:sched/core:

  e03dc9fa0663 sched/psi: Change update_triggers() to a 'void' function
  ...
  80cc1d1d5ee3 sched/psi: Avoid updating PSI triggers and ->rtpoll_total when there are no state changes
  3657680f38cd sched/psi: Delete the 'update_total' function parameter from update_triggers()

I rewrote the changelogs for readability.

Thanks,

	Ingo

===================>
From: Yang Yang <yang.yang29@....com.cn>
Date: Tue, 10 Oct 2023 16:41:07 +0800
Subject: [PATCH] sched/psi: Avoid updating PSI triggers and ->rtpoll_total when there are no state changes

When psimon wakes up and there are no state changes for ->rtpoll_states,
it's unnecessary to update triggers and ->rtpoll_total because the pressures
being monitored by the user have not changed.

This will help to slightly reduce unnecessary computations of PSI.

[ mingo: Changelog updates ]

Signed-off-by: Yang Yang <yang.yang29@....com.cn>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Cc: Peter Ziljstra <peterz@...radead.org>
Link: https://lore.kernel.org/r/202310101641075436843@zte.com.cn
---
 kernel/sched/psi.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index be853f227e40..79f8db0c6150 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -704,11 +704,12 @@ static void psi_rtpoll_work(struct psi_group *group)
 	}
 
 	if (now >= group->rtpoll_next_update) {
-		update_triggers(group, now, &update_total, PSI_POLL);
-		group->rtpoll_next_update = now + group->rtpoll_min_period;
-		if (update_total)
+		if (changed_states & group->rtpoll_states) {
+			update_triggers(group, now, &update_total, PSI_POLL);
 			memcpy(group->rtpoll_total, group->total[PSI_POLL],
 				   sizeof(group->rtpoll_total));
+		}
+		group->rtpoll_next_update = now + group->rtpoll_min_period;
 	}
 
 	psi_schedule_rtpoll_work(group,